Oathbound - Tracy Deonn

Legendborn Cycle Book 3

To protect her loved ones, Bree Matthews cuts off her ties and friends, paying a heavy price: an unbreakable bargain with the Shadow King, who trains her in exchange for her service as his protégé.

Concurrently, the remaining Scions face impending war with a fractured Round Table and a missing Selwyn. When Nick invokes an ancient law to address the High Council of Regents, he is imprisoned by the Order's Merlins amid hidden secrets and a wave of mysterious kidnappings and Merlins' deaths. The full synopsis can be found in the referenced sources.